1. Field of the Invention
The present invention relates to a photomask and a method of manufacturing thereof, and more particularly, to a photomask of high pattern resolution that can have a phase shift portion formed easily and with precision, and a method of manufacturing thereof.
2. Description of the Background Art
The remarkable advance in larger scale integration and miniaturization in semiconductor integrated circuits has also called for a drastic progress of miniaturization in circuit patterns formed on a semiconductor substrate (referred to as "wafer" hereinafter). The photolithography technique is particularly widely recognized as the basic technique in pattern formation. Various developments and improvements have been carried out regarding the photolithography technology. However, there is still a great pressure to scale the patterns to a higher density with the need of improvement in resolution.
The resolution R (nm) in photolithography using the reduction exposure method is typically expressed as R=K.sub.1 .multidot..lambda./(NA), where .lambda. is the wavelength of the employed light (nm), NA is the numerical aperture of the lens, and K1 is a constant depending on the resist process. It can be appreciated from the aforementioned equation that the resolution can be improved by reducing K1 and .lambda., and by increasing NA. In other words, the constant depending on the resist process should be lowered, as well as shortening the wavelength and increasing the NA. However, it is technically difficult to make improvements to the light source and the lens. Also, reduction in the wavelength of light and increase of the NA will cause a more shallow depth of focus of the light, resulting in the problem that the resolution is degraded. To date, various improvements are made to photomasks.
A reduction projection aligner for photolithography employing the reduction exposure method will be described hereinafter. FIG. 15 schematically shows a structure of the optical system of a conventionally used reduction projection aligner. Referring to FIG. 15, the reduction projection aligner includes a light source 10, a focused lens 11 beneath the light source 10 with a predetermined distance therebetween, a photomask 12 having a mask pattern formed therein which is to be written on a wafer, a reduction projection lens 13 for reducing light transmitted from the photomask 12, and a wafer stage 16 on which a wafer 15 to which the pattern is written is mounted. The wafer stage 16 is provided with a motor 14 for moving the wafer 15 to write a pattern on a predetermined position of the wafer 15 mounted on the wafer stage 16. In the reduction projection aligner of the above-described structure, light issued from the light source 10 is directed to a predetermined position on the wafer 15 mounted on the wafer stage 16 via the focused lens 11, the photomask 12, and the reduction projection lens 13, whereby the pattern formed on the photomask 12 is written. The photomask used in the above-described reduction projection aligner will be described in detail with reference to the drawings.
In FIG. 16, (a) shows a sectional view of a conventionally used photomask, (b) shows the amplitude of light right after passing the photomask, (c) shows the amplitude of light in the proximity of the wafer, (d) shows the light intensity on the wafer, and (e) a sectional view of a resist after being patterned using the photomask 20.
Referring to FIG. 16(a), the photomask 20 includes a glass substrate 21, and a mask pattern 22 formed of a metal such as chromium on the glass substrate 21. Light will not pass through the portion of the photomask 20 where the mask pattern 22 is formed. Therefore, the amplitude of the light directly after passing the photomask 20 is substantially 0 in the region corresponding to the mask pattern 22, as shown in FIG. 16(b). In the case where a fine pattern exceeding the aforementioned resolution (R) is to be transferred for the formation of a minute pattern, the light passing through the photomask 20 and the optical system therebeneath is subjected to light diffraction and interference to be enhanced in the overlapping region in adjacent pattern images in the proximity of the wafer, as shown in FIG. 16(c). The difference in intensity of light on the wafer will become smaller, as shown in FIG. 16(d), to result in a lower resolution. Therefore, there was a problem that a desired pattern configuration could not be obtained because sufficient patterning of a resist and the like could not be carried out.
A phase shift exposure method using a phase shift mask is proposed for obtaining a photomask without the above-described problems in, for example, Japanese Patent Laying-Open No. 57-62052 and Japanese Patent Laying-Open No. 58-173744. FIG. 17 shows a photomask using the phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744, in which (a) shows a sectional view of the photomask, (b) shows the amplitude of the light just passing the photomask, (c) shows the amplitude of the light in the proximity of the wafer after passing through the photomask, (d) shows the intensity of light on the wafer, and (e) shows the configuration of a sectional view of a resist after the resist is patterned using the above photomask.
Referring to FIG. 17(a), a transparent insulating film 23 formed of a silicon oxide film and the like is provided between predetermined mask patterns 22 formed on the surface of a glass substrate 21. The thickness of the transparent insulating film 23 is set so that the light passing through the transparent insulating film is inverted 180.degree. (in this specification, the portion inverting the phase of light by 180.degree. is defined as the "phase shift portion"). Regarding the amplitude of the light right after passing through the photomask, the light passing through the region where the transparent insulating film 23 is formed has its phase inverted 180.degree. with respect to that of the light passing through the portion where the transparent insulating film 23 is not formed, as shown in FIG. 17(b). Because the phases of the light are inverted with respect to each other in the portion where adjacent pattern images overlap, as shown in FIG. 17(c), the light will cancel each other by the interference effect. The difference in light intensity will be sufficient on the wafer to improve the resolution, as shown in FIG. 17(d). Thus, when patterning was carried out of a resist using the above-described phase shift mask, the patterning accuracy was improved, as shown in FIG. 17(e). In the present specification, a photomask including a phase shift portion is defined as a "phase shift mask".
Although such a phase shift mask is very efficient for a regular pattern, it was difficult to apply it to an arbitrary pattern. Thus, a phase shift mask was proposed which is applicable to the formation of an arbitrary pattern. FIG. 18 shows such a phase shift mask disclosed at the IEDM Conference in 1989, in which a sectional view thereof, the light intensity on a wafer where photolithography is carried out using this phase shift mask, and the manufacturing steps thereof, are shown.
Referring to FIG. 18, the phase shift mask includes a glass substrate 21, a mask pattern 22 formed on the glass substrate 21, and a resist film 24 formed on the mask pattern 22. The width of the resist mask 24 is larger than that of the mask pattern 22, in which the difference in the width (edge enhancement width) 25 serves as the phase shift portion. In the phase shift mask of the above-described type, light passing through the proximity of the edge of the mask pattern 22 has its phase inverted by 180.degree.. Light each having an opposite phase will overlap each other beneath the proximity of the edge of the mask pattern 22. Therefore, the light in the proximity of the edge of the pattern image will cancel each other on account of interference to increase the difference in light intensity on the wafer. As a result, a favorable resolution for an arbitrary pattern could be obtained.
However, such a phase shift mask had a problem that will be described hereinafter. The method of manufacturing this phase shift mask will first be described, followed by the problem thereof, with reference to FIGS. 19 and 20.
Referring to FIG. 18(a), a metal film, for example a chromium film 22a, is formed on a transparent glass substrate 1 by a sputtering method. An electron beam (referred to EB hereinafter) resist is applied all over the chromium film 22a to be subjected to thermal treatment, followed by depicting a desired pattern with an EB writing apparatus. Then, developing is carried out to form a resist pattern 24. Using this resist pattern 24 as a mask as shown in FIG. 18(c), the chromium film 22a is etched anisotropically or isotropically to result in a mask pattern 22. Then, using the same resist pattern 24 as a mask, the sidewall of the mask pattern 22 is etched by isotropic etching, for example wet etching. Thus, a mask pattern 22 will be formed that has its edge removed by the edge enhancement width 25.
This phase shift mask had the following problems due to its formation carried out in the above-described manner. The proximity of the edge portion of the resist pattern 24 must be thick enough to invert the phase of the light passing through the resist pattern 24 in order to function as a phase shift portion. However, the resist pattern 24 is used as a mask in the etching step for forming the mask pattern 22, resulting in decrease in thickness. Even if a predetermined film thickness of t was required of the resist pattern 24, only a film thickness of t1 could be obtained due to the film decrease in the etching process, as shown in FIG. 19. There was a possibility that the function of the phase shift portion could not be sufficiently provided. Accurate control of a film thickness to obtain a predetermined film thickness sufficient for a phase shift portion was also not easy.
With the structure of a phase shift mask where the mask pattern 22 is sandwiched, as shown in FIG. 18(d), it was difficult to control precisely the dimension of the edge enhancement width 25. There may be some cases where only a width of W1 of the mask pattern could be obtained with respect to a desired width of W on account of overetching of the sidewall. There was variation in the edge enhancement width 25, resulting in a problem that a transfer of a pattern according to the design could not be carried out. The mask pattern 22 and the resist pattern 24 of the photomask of FIG. 18 had a concaved and convexed configuration, so that contaminants could not be thoroughly removed in the cleaning process of the photomask. Contaminants remaining in the concaved and convexed portion of the photomask resulted in a problem that the pattern formation after the transfer was degraded.
Taking into consideration the foregoing, various improvements of the photomask have been proposed to carry out accurate control of the film thickness and width of the phase shift portion and to reduce the concaved-convexed stepped portion on the substrate. Photomasks disclosed in Japanese Patent Laying-Open Nos. 4-6557, 4-40455, and 4-3412 will be described hereinafter with reference to FIG. 21(a), FIG. 21(b), and FIG. 21(c), respectively.
Referring to FIG. 21(a), the phase shift mask disclosed in Japanese Patent Laying-Open No. 4-6557 has a mask pattern 22 formed on a glass substrate 21. A typical material of the mask pattern 22 includes Cr, MoSi, Si and the like. By subjecting this mask pattern to thermal oxidation, an oxide film 26 is formed to cover the mask pattern 22. The proximity of the periphery of the mask pattern 22 in the oxide film 26 functions as the phase shift portion. It is possible to control the thickness of the phase shift portion more accurately than a conventional one by appropriately adjusting the thermal oxidation condition in the present phase shift mask. Furthermore, the problem of contaminants remaining in the photomask can be avoided effectively because the stepped portion of the substrate and the oxide film 26 is not so complicated.
The phase shift mask of FIG. 21(b) has an opaque mask pattern 22 formed on a glass substrate 21 such as of crystal. Using a CVD method, a transparent film 27 is deposited on the glass substrate 21 and the mask pattern 22. The thickness of the mask pattern 22 and the transparent film 27 is adjusted so that the periphery of the mask pattern 22 is thick enough to function as a phase shift portion. Because a phase shift portion 27a can be formed just by depositing a transparent film, the control of film thickness can be carried out in accuracy in comparison with a conventional one. Also, the stepped portion in the phase shift mask can be reduced.
The phase shift mask of FIG. 21(c) has a film formed of ITO, Ta and the like serving as an etching stopper film 29 provided on a glass substrate 21. A mask pattern 22 is formed on the etching stopper film 29. A sidewall 28 formed of a silicon oxide film, for example, is provided at the sidewall of the mask pattern 22. This sidewall 28 functions as the phase shift portion. The sidewall 28 is formed by providing a silicon oxide film by a CVD method on the mask pattern 22 and the substrate 21, followed by anisotropic etching, referring to FIG. 21(b). By leaving the sidewall 28, it is possible to control the thickness of the phase shift portion by adjusting the thickness of the mask pattern 22. It is therefore possible to control more accurately the film thickness of the sidewall 28 which functions as the phase shift portion, and to reduce the concaved and convexed stepped portion of the photomask.
Thus, the three improved examples of a photomask allowed a more precise control of a film thickness and reduction in the stepped portion, so that the resolution could be improved in comparison with a conventional one.
However, the above-described improved examples had the following problems. In the phase shift mask of the FIG. 21(a), the formation of the oxide film 26 including the phase shift portion was carried out by thermal oxidation. This means that a thermal treatment of high temperature was applied, leading to a possibility of thermal expansion which generates thermal distortion in the mask pattern 22 formed on the substrate 21. If thermal distortion is generated, a mask pattern in conformity with the design could not be obtained, resulting in degradation in the dimension and position accuracy of the pattern. Further more, this phase shift mask had a relatively wide width of the portion functioning as the phase shift portion. Therefore, the width of the portion where light intensity is 0 is increased when the pattern is transferred to the wafer. Therefore, there was a problem that the mask pattern had to be formed taking into consideration the expansion of the portion at the design stage.
In the phase shift mask of FIG. 21(b), the transparent film 27 was formed using a CVD method to result in thermal distortion in the mask pattern 22, as in the above case. Therefore, there was a problem that the dimension and position accuracy of the pattern was degraded. In the phase shift mask of FIG. 21(c), if the silicon oxide film was deposited by a CVD method prior to the formation of the sidewall 28, thermal distortion was generated in the mask pattern 22, as in the above cases. Furthermore, this phase shift mask had an etching stopper film 29 formed on the substrate 21. Therefore, light transmittance is reduced by approximately 80% in comparison with the case where there is no etching stopper film 29, resulting in the problem of reduction in the intensity of the transmitted light. The etching stopper film 29 also had a problem that it has low tolerance to chemicals. More specifically, it was easily damaged by solutions such as alkaline based types in the cleaning process. There was a possibility of the etching stopper film 29 being removed after the cleaning step where the etching stopper film 29 is exposed. In this case, a portion of the etching stopper film 29 will remain beneath the sidewall 28 to modify the film thickness of the sidewall 28. More specifically, the film thickness of the phase shift portion may be changed.